The present invention relates to a synchronous control method and an apparatus therefor which are suitable for a case where a work which is held between two opposed spindles in an opposed spindle lathe is synchronously rotated, the two spindles being opposed to each other and rotatably provided.
In such an opposed spindle lathe having two spindles which are opposed to each other and rotatably provided, when a work is subjected to adjustable-speed rotation or cutting work while being held between the two opposed spindles, the two spindles must be synchronously controlled so that no excessive torsional torque occurs in the work. This synchronous control can be realized by a first method of precisely controlling both spindles so that torsional torque is no greater than an allowable value or by a second method disclosed in Japanese Laid-open Patent Application Nos. 2843/1989 and 2844/1989.
In the first conventional method, when both spindles and the work have a high rigidity, torsional torque excessively occurs in the work even if only a slight deviation occurs in synchronization, and power is transmitted from one spindle driving servo motor to the other spindle driving servo motor. This causes unnecessary power running or regenerative operation of both servo motors and thus causes an unnecessary current to flow to both servo motors and the driving apparatus therefor, resulting in the occurence of a trouble in the control.
In the second conventional method, one spindle driving servo motor is rotated with a given torque, and the other spindle driving servo motor is rotated with a torque which is smaller than the given torque. However, since this method is based on the assumption that there is a difference in torque between both spindles, the torsional torque occurs in the work, and, particularly, there is a problem with respect to the precision of synchronous control when the work has a low level of torsional rigidity.